In the LTE (Long Term Evolution) Advanced of the 3GPP (Third Generation Partnership Project), OFDMA (Orthogonal Frequency Division Multiplexing Access) using MU-MIMO (multi-user multiple-input multiple-output) has been proposed. In MU-MIMO downlink transmission, a base station is able to not only communicate with multiple mobile terminals (UE, user equipments), but also to transmit different data streams (layers) simultaneously to a mobile terminal.
In addition, in LTE Advanced, a reception technique for mobile terminals called “interference rejection combining” has been discussed. Interference rejection combining (IRC) is a technique for downlink communication, in which a mobile terminal gives weights to signals obtained by reception antennas so as to suppress interference with the desired radio wave beam from the visited base station (desired base station) caused by interfering radio wave beams from interfering base stations at the mobile terminal. IRC improves the reception quality of desired signals on a desired radio wave beam, especially in a case in which a mobile terminal 4 is located near the boundary of a visited cell 1a (cell of the desired base station 1) and receives strong interfering radio wave beams from another base station 2 (interfering base station) as shown in FIG. 1. In FIG. 1, reference symbol 2a denotes the cell of the interfering base station 2. In addition, in FIG. 1, a general shape of a beam 1b generated at the desired base station 1, and a general shape of the beam 2b generated at the interfering base station 2 are illustrated. A part of the beam 2b generated at the interfering base station 2, i.e., a part of a beam for downlink channels for other mobile terminals (for example, a mobile terminal 5) causes an interfering signal for the mobile terminal 4.
IRC is described in, for example, Patent Document 1, Non-patent Document 1, and Non-patent Document 2. In IRC, the mobile terminal gives weights to multiple signals obtained by reception antennas so as to suppress interference with the desired radio wave beam caused by interfering radio wave beams, and separates a signal destined for the mobile terminal from signals destined for other mobile terminals among multiple signals derived from radio waves received at the multiple receiving antennas with the use of the reception weights.
In recent years, a technology has been proposed in which multiple radio base stations collaborate to adjust radio wave beams for controlling interfering radio wave beams received at mobile terminals. For example, Non-patent Document 3 discloses interference alignment (IA). In interference alignment, multiple radio base stations share downlink channel characteristic information on multiple mobile terminals, and multiple interfering base stations for each single mobile terminal coordinates to perform in-phase addition of interfering radio wave beams. As a result, the number of interfering radio wave beams arriving at the mobile terminal is reduced. The mobile terminal can eliminate or reduce the adverse effect of interference by, for example, using IRC.
Furthermore, 3GPP has discussed the application of a technology called Coordinated Multiple Point Transmission and Reception (CoMP) for LTE Advanced (see Clause 8, Non-Patent Document 4, for example).
Downlink CoMP is a technology in which multiple radio base stations coordinate with each other in order to send data signals to mobile terminals. Downlink CoMP is roughly divided into Coordinated Scheduling/Beamforming (CS/CB) and Joint Processing (JP).
In JP in downlink CoMP, multiple coordinating radio base stations share data signals destined for all UEs connected to these radio base stations, in addition to channel quality information and the like. These radio base stations mutually coordinate to send the data signals to the UEs. For example, two or three radio base stations send data signals to a single UE at the same time.
In CS/CB in downlink CoMP, a data signal exists only in a transmission-source radio base station to which a transmission-destination mobile terminal is connected. In CS/CB, however, information (channel quality information and the like) about all mobile terminals connected to a radio base station serving as the data-signal transmission source, and to one or more other radio base stations that coordinate with the radio base station, is shared by these radio base stations, and these radio base stations mutually coordinate to perform scheduling or beam forming in order to send a data signal to each mobile terminal. In other words, each of a plurality of coordinating radio base stations sends data signals to mobile terminals in the cell of the radio base station, and the plurality of coordinating radio base stations share channel quality information and the like related to the mobile terminals in order to perform appropriate scheduling or appropriate beam forming for data transmission.
In CS/CB in downlink CoMP, beam forming has been proposed in which a radio base station directs main beams to mobile terminals to which data signals are sent, and directs nulling beams to mobile terminals that communicate with other radio base stations. For this purpose, multiple radio base stations share downlink channel characteristic information of multiple mobile terminals. As a result, each mobile terminal receives a strong desired radio wave beam from its desired radio base station and weak interfering radio wave beams from interfering radio base stations.